The present invention relates in general to a control apparatus for maintaining constant the pulling force of a hydraulic cylinder.
It is known in the art to use control means of this type to maintain a predetermined pulling force in particular for cables or ropes. The rope is guided around a pulley to form a loop which pulley is connected to a hydraulic cylinder for adjustment. Fast movements of the cable are compensated for by the cylinder which piston extends or, respectively, retracts the pulley to decrease or increase the length of the cable loop. Under the condition that the pulley force or tie load of the rope shall be kept constant during the movements the pressure in the working chamber of the cylinder must be maintained constant independent of the position of the guide pulley.
Preferably, control means of this type are used on vessels for drawing cables or ropes which are connected to other stationary or movable devices. In addition to the constant pulling force control, a cable drum is provided which is driven to compensate for slow movements of the cable or rope.
According to a prior art control called Mooring a hydrostatic machine having a variable displacement volume is operated by being pressure controlled. The machine either operates as a pump or as a motor depending on whether control liquid is sucked into the working chamber in extending the piston or is displaced from the working chamber in retracting the piston. A substantial drawback of this control apparatus is the high power loss.
It is further known to connect the working chamber of the hydraulic cylinder to an accumulator. Accordingly, control liquid is either fed into the accumulator or taken from the accumulator. Since the pressure depends on the size and characteristics of the accumulator a constant pulling force cannot be obtained. Further, as soon as the tractive power of the rope and thus the pressure changes, this results in difficulties.